Coated articles are known in the art for use in window applications such as insulating glass (IG) window units, vehicle windows, and/or the like. In certain situations, designers of coated articles often strive for a combination of high visible transmission, substantially neutral color, low emissivity (or emittance), and blockage of undesirable radiation such as infrared (IR) radiation to prevent undesirable heating of a building interior or the like. High visible transmission for example may permit coated articles to be more desirable in certain window applications, whereas low-emissivity (low-E), low SHGC (solar heat gain coefficient), and low SF (solar factor, or g-value) characteristics permit coated articles to block significant amounts of undesirable radiation so as to reduce, for example, undesirable heating of vehicle or building interiors.
Solar factor (SF, or g-value), calculated in accordance with DIN standard 67507, relates to a ratio between the total energy entering a room or the like through a glazing and the incident solar energy. Thus, it will be appreciated that low SF values are indicative of good solar protection against undesirable heating of rooms or the like protected by windows/glazings. For example, a low SF value is indicative of a coated article (e.g., IG unit such as a double glazing) that is capable of keeping a room fairly cool in summertime months during hot ambient conditions.
While low SF values are typically desirable for coated articles such as IG window units, the achievement of low SF values typically comes at the expense of visible transmission and/or coloration. It is often desirable, but very difficult, to achieve a combination of a high visible transmission and a low SF value for a coated article such as an IG window unit or the like. In this regard, the ratio between visible transmission (Tvis) and SF is sometimes referred to as “selectivity.” In other words, the “selectivity” of a coated article is defined by Tvis/SF.
High selectivity (Tvis/SF) values are indicative of a combination of high visible transmission and low SF, and are thus often desirable. Unfortunately, high selectivity (Tvis/SF) values have heretofore been difficult to achieve in certain situations.
For example, an object of glazings described in U.S. Pat. No. 6,673,427 to Guiselin is to achieve the “highest possible” selectivity (i.e., Tvis/SF). In this regard, see the '427 Patent at column 1, lines 54-60. Given this goal of achieving the highest possible selectivity (i.e., Tvis/SF), glazings according to the '427 Patent were able to achieve a selectivity of about 1.6 to 1.7 in a double glazing (see the '427 Patent at column 7, lines 3-5). In particular, Example 3 of the '427 Patent achieved a selectivity of about 1.67 whereas Example 4 of the '427 Patent achieved a selectivity of about 1.61 as evidenced by Table 2 of the '427 Patent (e.g., for Example 4, 61/38=1.605).
While higher selectivities are sometimes achievable, they have come at the expense of higher SF values and/or undesirable coloration at normal and/or off-normal viewing angles such as 45 degrees. For example, Examples 1 and 2 of U.S. Pat. No. 5,595,825 to Guiselin used triple-silver coatings to allegedly achieve selectivity values in double glazings of 1.97 and 1.82, respectively. However, the coatings of the '825 Patent required the use of three separate silver layers at particular thicknesses to achieve such selectivity values, at the expense of higher SF values of 30 and 34 respectively. Such high SF values may sometimes be undesirable in certain example non-limiting instances since they are indicative of coatings that in certain situations cannot block sufficient radiation from reaching a building or vehicle interior. Too, in certain example instances, the requirement of three separate silver layers may be undesirable in certain example non-limiting situations in that such coatings are more costly and burdensome to manufacture and may be more susceptible to yield problems. Furthermore, it is unclear from the '825 Patent whether the coatings thereof realize significant color shifts upon change in viewing angle and/or undesirable coloration.
U.S. 2003/0150711 to Laird, the disclosure of which is hereby incorporated herein by reference, discloses a coating having the following layers oriented from the glass substrate outwardly:
Glass SubstrateThickness (Å)TiO2200ZnO90Ag130NiCrOx30SnO2680ZnO90Ag168NiCrOx30SnO2125Si3N4220
While the aforesaid coated article of U.S. 2003/0150711 achieves excellent results in many respects, when used in the context of an IG window unit (or double glazing), it typically achieves a selectivity (Tvis/SF) of about 1.7 or so. Higher selectivity values are often desirable in certain example non-limiting situations.
One approach in the art to improving color at high viewing angles in double-silver coatings (i.e., coatings with a pair of silver layers) is to make the top silver layer significantly thicker than the bottom silver layer. For example, see U.S. Pat. No. 6,673,427 to Guiselin, discussed above. A goal of such coatings is to prevent a color shift from blue-green to red with a change in viewing angle. Such color changes to red are easily picked up by observers and are sometimes considered to be objectionable. However, such designs may sometimes suffer due to their need to position the NIR reflectance increase well outside the visible spectra from a limitation with respect to the SHGC (or SF) achievable. In other words, due to the need to position the NIR reflectance increase well outside the visible spectra in such coatings in order to prevent a color shift to red at angle, low SF and/or SHGC values are not easily achievable for a given visible transmittance (the same may apply for maximum Light to Solar Gain—LSG—ratio) in certain example non-limiting coatings where the top silver layer is significantly thicker than the bottom silver layer as in the '427 Patent. Note the SF values of 38-42 in Examples 1-8 of the '427 Patent, which in certain example non-limiting situations are rather high.
In view of the above, it will be apparent to those skilled in the art that there exists a need for coated articles which are capable of providing one or more of high selectivity values, low SF values, substantially neutral color at normal and/or off-axis viewing angles, and/or low emissivity (or emittance).